Electrochromic compounds exhibit a reversible color change when the compounds gain or lose electrons. Electrochromic devices that exploit the inherent properties of electrochromic compounds find application in large area static displays and automatically dimming mirrors, and are well known. Electrochromic display devices create images by selectively modulating light that passes through a controlled region containing an electrochromic compound. A multitude of controlled electrochromic regions may individually function as pixels to collectively create a high resolution image. Typically, these display devices contain a reflective layer underneath the electrochromic compound, respective to the viewer, for reflecting light allowed to pass beyond the electrochromic region. Simply put, the electrochromic pixel acts as a shutter either blocking light or allowing light to pass through to the underlying reflective layer.
A typical prior art electrochromic display device 10, as shown in FIG. 1, includes a base substrate 10, typically glass or plastic, which supports a transparent conductor layer 20, which may be, for example, a layer of fluorine doped tin oxide (FTO) or indium doped tin oxide (ITO). A nanoporous-nanocrystalline semi-conducting film 30, (herein referred to simply as a nano-structured film 30), is deposited, preferably by way of screen printing with an organic binder, on the transparent conductor 20. The nano-structured film is typically a doped metal oxide, such as antimony tin oxide (ATO). Optionally, a redox reaction promoter compound is adsorbed on the nano-structured film 30. An ion-permeable reflective layer 40, typically white titanium dioxide (TiO2), is optionally deposited, preferably by way of screen printing with an organic binder followed by sintering, on the nano-structured film 30.
A second substrate 50, which is transparent, supports a transparent conductor layer 60, which may be a layer of FTO or ITO. A nano-structured film 70 having a redox chromophore 75, typically a 4,4′-bipyridinium derivative compound, adsorbed thereto is deposited on the transparent conductor 60, by way of a self-assembled mono-layer deposition from solution.
The base substrate 10 and the second substrate 50 are then assembled with an electrolyte 80 placed between the ion-permeable reflective layer 40 and the nano-structured film 70 having an adsorbed redox chromophore 75. A potential applied across the cathode electrode 90 and the anode electrode 100 reduces the adsorbed redox chromophore 75, thereby producing a color change. Reversing the polarity of the potential reverses the color change. When the redox chromophore 75 is generally black or very deep purple in a reduced state, a viewer 110 perceives a generally black or very deep purple color. When the redox chromophore 75 is in an oxidized state and generally clear, a viewer 110 will perceive light reflected off of the ion-permeable reflective layer 40, which is generally white. In this manner, a black and white display is realized by a viewer 110.
Electrochromic display devices such as the one described above are described in greater detail in U.S. Pat. No. 6,301,038 and U.S. Pat. No. 6,870,657, both to Fitzmaurice et al., which are herein incorporated by reference.
The electrochromic display 10 shown in FIG. 1 is a pixilated display, having individual image elements, (i.e. pixels A, B, and C). The potential applied to each pixel A, B, and C is provided by a dedicated routing track in the transparent conductive layer 60. Each pixel A, B, and C is therefore directly driven; a voltage applied to pixel A will not interfere with pixels B or C. In order to create a large electrochromic display capable of displaying high resolution images, a large number of pixels is required, and therefore a large number of direct drive routing tracks. For a typical computer monitor having millions of pixels, fabricating millions of direct drive routing tracks is impractical.
To reduce the complexity of providing each pixel with its own direct drive routing track, an active matrix may be used. In an active matrix, each pixel has an active component for electrically isolating each pixel from all other pixels and for matrix addressing of each pixel. FIG. 2A is a schematic illustration of an active matrix 200 for controlling a plurality of pixels addressed in rows R1 . . . R4 and columns C1 . . . C7. A multitude of active devices 210, typically transistors, are located at the intersection of each row and column. Referring to FIGS. 2A and 2B, each active device 210 includes a gate electrode 220, a source electrode 230 and a drain electrode 240. The cathode 250 of each pixel 260 is electrically connected to the drain electrode 240 of the active device 210. The anode 270 of the pixel 260 is commonly connected across all pixels.
To write data to a desired pixel 260, for example the pixel 260 at the intersection of row R2 and column C2, a row signal is applied to row R2 to activate the active device 210, while a different row signal is applied to all other rows (i.e. rows R1, R3, and R4) to ensure active devices 210 in these rows are kept inactive. A column signal is then applied on column C2 to write data to the pixel 210. Typically, an entire row of pixels will be updated simultaneously by writing data to each pixel in a selected row at the same time. In this manner, a large number and high density of pixels may be individually controlled while maintaining electrical isolation of each pixel.
Typically, an active matrix is constructed from thin film transistors (TFTs). The fabrication of TFTs is well known in the art and includes the deposition of opaque metal layers on an insulative substrate. Therefore, TFTs are not transparent or translucent. Furthermore, in order to achieve optimal switching times and performance in an electrochromic display of the kind described above, the drain of each TFT must be on the cathode side of the display (i.e. on the side contained the nano-structured film with adsorbed viologen). Achieving active control of pixels A, B, and C in the electrochromic display 10 therefore requires placement of opaque TFTs on the front plane of the display, with respect to the viewer 110. This is disadvantageous as opaque TFTs diminish the reflectivity of the display, reduce pixel aperture, and adversely affect contrast ratio and apparent brightness of the display.
A further complication is that the relationship between transistor size and the switching speed of an electrochromic pixel is inversely proportional; the larger the transistor, the faster the switching time. Achieving video speed switching times, which are on the order of tens of milliseconds, therefore requires a large transistor relative to the area of the electrochromic pixel.
To illustrate, referring to FIG. 3, an electrochromic display 300 including a TFT active matrix 120 for individually addressing each pixel A, B, and C is shown. Each pixel A, B, and C is controlled by a transistor 120A, 120B, and 120C, respectively. Each opaque transistor 120A, 120B, and 120C blocks a portion of the pixel it is controlling, (A, B, and C, respectively). Achieving fast switching times increases transistor size, thereby decreasing the visible pixel area and the reflectivity, contrast ratio, and brightness of the display 300.
Therefore, an active matrix electrochromic display without the disadvantages of the prior art is desired.